As is well known to those skilled in the art, solid carbonaceous materials including coals of high and low rank, may be subjected to various processes to convert at least a portion of the carbon present in the solid to a liquid form; and these processes for hydrogenation of coal are commonly termed coal liquefaction processes.
In these processes, finely powdered coal, typically of size such that at least 100 w% passses through a 40 mesh US Standard sieve is contacted (in the form of a slurry) with hydrogen at 350.degree. C.-600.degree. C., say 450.degree. C. and 1000-2500 psig, say 2000 psig to form hydrocarbons characterized by increased hydrogen content. Illustrative of processes for upgrading coal are those disclosed in U.S. Pat. No. 2,221,886, U.S. Pat. No. 2,860,101, and U.S. Pat. No. 3,341,447.
Product hydrocarbon liquids are separated by distillation leaving an extremely hydrophobic solid typically characterized as follows:
TABLE ______________________________________ Property Value ______________________________________ carbon (w %) 65.2 density g/cc 1.44 boiling point above 700.degree. C. melting point 200-300.degree. C. ash (w %) 27.85 particle size mean 71.4 microns range 30-220 microns ______________________________________
It is desirable to use this coal liquefaction residue as a charge to a gasification reaction i.e. to convert it to a synthesis gas (containing carbon monoxide and hydrogen) by partial combustion; but this has proven to be difficult because of the problems encountered during feed preparation. The synthetic coal liquefaction is a composition which has properties totally unlike other carbonaceous materials; and these render it particularly and uniquely difficult to handle.
It is found that if coal liquefaction residue be subjected to grinding and mixing with water in an attempt to form a slurry suitable for use as feed to gasification, the slurry is characterized by problems, the principal one of which is creaming.
Creaming, as the term is used in this specification, refers to the separation of phases in a system containing an aqueous medium and a finely divided hydrophobic solid medium, the latter forming a supernatant phase above a large body of liquid containing solids.
Creaming is distinguished from other phenomena which may be present in two phase systems including the following:
(i) Foaming refers to the formation of a gas-liquid mixture of low density adjacent to the surface of a lower body which may for example be a uniform mixture of liquids and solids. A foam is characterized by the presence of a large proportion of gas phase and very little (usually no) solid phase; by a low density; and by the fact that it may frequently be eliminated or minimized by addition of agents which lower the surface tension of the liquid component of the foam. Elimination of foaming does not per se mean elimination of creaming.
(ii) Emulsification refers to the formation of a mixture of particles of one liquid with a second liquid. In commercial practice, one liquid is invariably water and the other is an oil. Thus the two common types of emulsions are oil-in-water (O/W) and water-in-oil (W/O). In certain instances the presence of solid particles may stabilize emulsions by collecting at the oil-water interface and armoring the phase for which the solid has greater affinity. Addition of surface-active agents can enhance or destroy the stability of the emulsion, but it has no effect on the creaming phenomenon.
It is particularly to be noted that procedures which solve other problems do not necessarily solve the problem of creaming. For example, it is possible to utilize a system which provides satisfactory emulsifying or dispersing properties but which fails to solve the problem of creaming.
It is an object of this invention to provide a process for eliminating creaming in a disperse slurry of coal liquefaction residue in aqueous medium. Other objects will be apparent to those skilled in the art.